Human eye can be treated as an optical instrument. Light rays coming from the object to be seen enter the eye through Cornea and fall on the eye lens through the pupil of the eye.
(a) Cornea:-The transparent spherical membrane covering the front of the eye.
(b) Iris:-The coloured diaphragm between the cornea and lens.
(c) Pupil:-The small hole in the iris.
(d) Eye lens:-Its is a transparent lens made of jelly like material.
(e) Ciliary muscles:-These muscles hold the lens in position.
(f) Retina:-The back surface of the eye.
(g) Blind spot:-The point at which the optic nerve leaves the eye. An image formed at this point is not sent to
(h) Aqueous humour:-A clear liquid region between the cornea and the lens.
(i) Vitreous humour:-The space between eye lens and retina is is filled with another liquid called Vitreous
Accomodation:-The ability of the eye to focus both near and distant objects, by adjusting the focal length, is called the accommodation of the eye or the ability of the ciliary muscles to change the focal length of the eye lens is called accommodation.
Near point of the Eye: It is the closest point at which an object can be seen clearly. For normal eye, the near point lies at 25 cm from the eye(least distance of distinct vision)
Defects of vision:
Causes of Myopia:
2. Decrease in focal length of eye lens when the eye is fully relaxed.
Remedy: To correct a myopic eye, the person has to wear spectacle with a concave lens of suitable focal length. (i.e.) the focal length of concave lens is equal to the distance of the far point of the myopic eye.
Hypermetropia (or) Long sightedness: It is that defect of a human eye by virtue of which it can see clearly the objects lying at large distances from it but the nearby objects cannot be seen clearly.
Remedy: To correct a hypermetropic eye, the person has to wear spectacle with a convex lens of suitable focal length.
The defect by which the person is notable to differentiate horizontal and vertical position, is called astigmatism. It can be rectified by using cylindrical lenses.
Visible spectrum: The band of seven colours obtained due to the dispersion of white light is called a visible spectrum (VIBGYOR).
Application of dispersion:
RAINBOW: It is a concentric coloured circular arc in the sky when the sun rays fall on rain drops during or after a shower. To watch a rainbow observer must stand with his back towards the sun.
The formation of rainbow is due to the dispersion of white light from the sun and the phenomenon of total internal reflection of light from the water droplets suspended in the air after a shower
Total internal reflection: When light travels from denser to rarer medium incident on the interface separating two media at an angle larger than a particular angle called critical angle (which is a constant for a given pair of media) will be totally reflected back into the denser medium
• Atmosphere layers having different temperature hence optical density of different layers are different. As we go upwards, the atmosphere becomes rarified. So the Refractive Index decreases as we go upwards. So, the light from the sun or any other star undergoes refraction.
Is µ 1/λ4
The sun looks reddish at the time of sun rise and sun set